feat(viewer): UI-02 heart particles, interaction notices, share and download

Task 5 (V05 §5.4/§6): decorative heart particles on the lightbox stage
(local like clicks + others' WS bursts for the current photo), compact
cross-photo interaction notices above the action bar (click to locate),
navigator.share-first share orchestration with clipboard fallback for
album and photo deep links (lang param), and download failures attributed
to the photo that triggered them. Self-echo dedup stays in the realtime
router (own batches never produce particles/notices); particles cap at
12 with visibility/unmount cleanup and are suppressed under reduced
motion; notices cap at 3 with ~4s auto-dismiss and throttled SR summary.

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/viewer/src/download/useDownload.ts

@@ -68,6 +68,9 @@ export function useDownload(inputs: DownloadInputs) {
   /** 失败文案键（''=无失败）：默认通用失败；DOWNLOAD_DISABLED 给专属文案
    * （T15 漏译修复——相册中途关闭下载开关时观众可理解的确定性提示）。 */
   const failed = ref<'' | 'download.failed' | 'errors.downloadDisabled'>('');
+  /** 失败归属照片（UI-02 §5.4）：异步错误绑定触发下载的 photoId——大图内切图
+   * 后错误提示只在切回该照片时展示，不张冠李戴到当前照片。 */
+  const failedFor = ref<string | null>(null);
   /** 微信引导浮层可见（点击下载时处于微信内置浏览器）。 */
   const wechatGuide = ref(false);
   const copyNotice = ref<'' | 'copied' | 'failed'>('');
@@ -95,6 +98,7 @@ export function useDownload(inputs: DownloadInputs) {
     if (photoId === null || busy.value) return;
     busy.value = true;
     failed.value = '';
+    failedFor.value = null;
     try {
       let credential = await getPhotoDownload(inputs.token.value, photoId, variant);
       if (isExpired(credential.expiresAt)) {
@@ -108,6 +112,7 @@ export function useDownload(inputs: DownloadInputs) {
         error instanceof ApiError && error.code === ErrorCodes.DOWNLOAD_DISABLED
           ? 'errors.downloadDisabled'
           : 'download.failed';
+      failedFor.value = photoId; // 异步错误归属触发时的照片（§5.4）
     } finally {
       busy.value = false;
     }
@@ -140,6 +145,7 @@ export function useDownload(inputs: DownloadInputs) {
   return reactive({
     busy,
     failed,
+    failedFor,
     wechatGuide,
     copyNotice,
     canDownload,